Field of the Invention
The invention relates to a method for operating a Coriolis mass flowmeter wherein the Coriolis mass flowmeter has at least one measuring tube, at least one oscillation generator, at least two oscillation sensors and at least one evaluation unit, wherein the oscillation sensors and/or oscillation generator include each at least one permanent magnet and each at least one coil, wherein the oscillation generator excites the measuring tube to oscillation, wherein the oscillation sensors detect the oscillations of the measuring tube, wherein the temperature of the measuring tube is determined and wherein the evaluation unit processes the detected oscillation signals and determines state variables of the Coriolis mass flowmeter.
Description of Related Art
Coriolis mass flowmeters are known in a plurality of designs in the prior art. Mass flowmeters, which function using the Coriolis principle, have at least one oscillation generator, with which the measuring tube is excited to oscillation- or optionally also several measuring tubes that are excited to oscillation-, as well as at least two oscillation sensors, with which the desired oscillation or desired oscillations of the measuring tube is/are detected. The oscillation sensors are attached to the measuring tube at the inlet and the outlet. In the case that a medium having mass is flowing through the measuring tube and the measuring tube is set into oscillation, counteracting oscillation portions result at the inlet and outlet, which are determined by the oscillation sensors. In the case of no flow, the signals of both oscillation sensors are in-phase in the ideal case. The reason for oscillation portions that are increasingly different at the inlet and outlet with increasing mass flow is that there are differently directed Coriolis forces on the flowing medium at the inlet and outlet.
Oscillation generators from Coriolis mass flowmeters of the type mentioned above include a coil and a permanent magnet. The permanent magnet is, at any rate, arranged in the influencing portion of the coil so that the magnetic field generated during current feed of the coil interacts with the magnetic field of the permanent magnet and corresponding forces act on the coil and the permanent magnet. The amount of electric current with which the coils is impinged is decisive for the force that is transferred from the oscillation generator to the measuring tube and that excites the measuring tube to oscillation; the current feed of the coil generally occurs harmonically, and thus, also the application of force on the measuring tube. When the Coriolis mass flowmeter is viewed as a dynamic system with the input variable excitation force and the output variable measuring tube deflection- or a variable derived temporally therefrom such as measuring tube velocity-, then, at a known excitation force and a measured measuring tube velocity, information is available for identifying the transfer behavior of the dynamic system Coriolis mass flowmeter. If identification is carried out on the basis of a mathematical model, it is possible to define the parameters used in the model, such as the spring stiffness of the measuring tube, the damping of the measuring tube and the oscillating overall mass see, German Patent Application DE 10 2012 011 934 A1 and corresponding U.S. Patent Application Publication 2013/0338943 A1, as well as German Patent Application DE 10 2008 059 920 A1 and corresponding U.S. Pat. No. 8,104,361 B2.
The physical parameters and thus also the functional relationships, however, are often subject to further influences, which are taken into consideration in the ideal case, in order to achieve exact measuring results. For example, the spring stiffness of the measuring tube, among other things, is temperature-dependent. Methods are known to the applicant from the prior art, in which the measuring tube temperature is measured by a sensor attached to the measuring tube, in order, for example, to be able to take the temperature-dependence of the spring stiffness of the measuring tube into consideration.